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It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.

But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?

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  • $\begingroup$ @usernumber I don't understand how. Looking at reflecting telescope set ups in diagrams, it doesn't look like the mirror could act as an aperture $\endgroup$
    – XXb8
    Apr 21, 2020 at 14:28
  • $\begingroup$ If you use a refracting telescope with a lens twice as big, or ten times as big, you will still have diffraction. The diffraction in a refracting telescope is not caused by the size of the lens. $\endgroup$
    – usernumber
    Apr 21, 2020 at 14:29
  • $\begingroup$ @usernumber on this link page 18 blog.ulverstonvictoria.cumbria.sch.uk/physics/files/2016/02/… this is why I'm confused. It implied the lens itself is the aperture that causes diffraction. Is this true? If so, how can a mirror have the same effect? It's not exactly a 'gap'? $\endgroup$
    – XXb8
    Apr 21, 2020 at 14:32
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    $\begingroup$ Diffraction happens on apperture, thus the round opening (or however shaped) opening where light passes into the optics - whether it's a mirror or a lens is irrelevant. Don't confuse diffraction with refraction. $\endgroup$
    – planetmaker
    Apr 21, 2020 at 14:42
  • $\begingroup$ "Opening" would be better than "gap;" in any case try not to get hung up on word choice. Any objective of finite size will have diffraction inversely related to its size. $\endgroup$
    – Mike G
    Apr 21, 2020 at 16:41

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Diffraction has nothing to do with whether you are using a lens or a mirror in your telescope. Diffraction occurs for the same reason in reflecting telescopes as it does in refracting telescopes: it is passing through an aperture.

There is no threshold aperture diameter for which you have diffraction. No matter how big the aperture, you will have diffraction. No matter the shape of the aperture, you will also have diffraction. This happens on any aperture.

However, the bigger the aperture, the smaller the diffraction pattern, or Airy disc. In page 19 of the document you mention in the comments, there is a relation between the size of the aperture and the size of the Airy disc.

To clarify what is said in the first paragraph of page 18

If the telescope is replaced by one with a narrower objective, the images of the two stars would overlap too much and the observer would not be able to see them as separate stars.

If the two stars are replaced with two stars that are close enough together, the images of the two stars would overlap too much and in this case the observer would not be able to see them as separate stars either.

As a side note, refracting telescopes aren't necessarily smaller than reflecting telescopes. There are refracting telescopes with a lens more than a meter in diameter. A lot bigger than most amateur reflecting telescopes !

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  • $\begingroup$ This makes sense, but the question I was trying to ask was how the mirror acts as an aperture? This is the misconception that's stopping me from understanding everything else $\endgroup$
    – XXb8
    Apr 21, 2020 at 14:53
  • $\begingroup$ @XXb8 When light passes through an aperture, it is diffracted. If you have a cardboard tube with no optics in it, light will still be diffracted. $\endgroup$
    – usernumber
    Apr 21, 2020 at 14:58
  • $\begingroup$ So how would it diffract on a mirror? Around the edges of the mirror or does light spread after it's reflected? $\endgroup$
    – XXb8
    Apr 21, 2020 at 15:04
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    $\begingroup$ It doesn't diffract because of the mirror, it diffracts when it passes through the aperture at the opposite end of the telescope. The mirror has nothing to do with the diffraction. $\endgroup$
    – Jim421616
    Apr 22, 2020 at 3:46

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